Radio communication system

ABSTRACT

A radio communications system that utilizes multiple cells to provide radio communication services to a plurality of mobile stations. In the cells that service high concentrations of mobile stations within a very small geographic area, such as large office buildings, there are provided a plurality of radio frequency capsules within the geographic area serviced by the cell. In addition to having a base site for providing Broadcast Control Channel(BCCH), the cell contains multiple RF capsules for transmitting and receiving traffic channels as well as receiving Random Access Channel(RACH) bursts from a plurality of mobile stations in a portion of the geographic area serviced by the base site. The RF capsules do not provide a BCCH, rather the BCCH is provided by a microcell transceiver that is servicing the particular cell.

FIELD OF THE INVENTION

Generally, this invention concerns radio communication systems and moreparticularly cellular radio communication systems with use of RadioFrequency (RF) capsules within a standard cell.

BACKGROUND OF THE INVENTION

Traditional cellular systems break down a geographic area into groupingsof cells. Each cell provides communication services to mobile stationscontained within that cell. In order to provide these services, a basestation provides a broadcast control channel upon a predeterminedfrequency from which all mobile stations contained within the cell canlisten. In response, a mobile station can transmit a random accesschannel burst back to the serving base site where upon the base siteallocates a traffic channel for which the mobile station may communicateto the base site for providing communication services such as voice andor data services.

There is an inherent limitation on the number of mobile stations towhich a base site may provide communication services. This limitation isdetermined by the cell size, the frequency reuse pattern of the cell andits surrounding cells and the number of frequencies or traffic channelsallocated to the cellular system. In order to increase the capacityprovided by a cellular system the physical size of the cells may bereduced, thus, providing more cells within a particular system.

As cellular systems become more prevalent in our society and the usageof the systems increases, there is a need to provide more capacitywithin the cellular systems. When implementing cellular systems withinlarge buildings there are many sources of interference and also a largeconcentration of people that could use the system. Traditional systemsprovide many extremely small cells to provide service to all the userswithin the building. By providing this plurality of very small cells,there is often a need to hand over more frequently between the cellscreating a large amount of overhead communications between cells.Examples of such handover procedures may be found in the Global Systemfor Mobile communications (GSM) recommendations.

Additionally, these systems are extremely sensitive to interferencecaused by adjacent cells, consequently they are fixed geographically andmust be re-planned for each change in the dynamics of the system.

FIG. 1 is an illustration of a cellular communication system 100 thatprovides communication service in three distinct cellular cells 101, 103and 105. Each of the three cell sites include a base station 107, 109and 111 that transmit a broadcast control channel for that particulargeographic area defined by the cell. Additionally, each mobile station113, 115 and 117 contained within the respective cells transmit RandomAccess Channel (RACH) bursts back to the base site 107, 109 and 111. Ifthe physical size of these cells shrink to a few meters in diameter, thecomplexity in handing off between a first cell 101 and a second cell 103become more frequent and more cumbersome in the system architecture.Additionally, cell overlap creates interference problems which areimmeasurable by the conventional system and therefore discouraged. Thusin order to properly design such a small system the placement of thecells must be carefully planned and any changes such as office reconfiguration tearing down walls or just moving furniture within theoffices can create enough overlap for which a cellular re plan isrequired creating a large amount of headache for system implementers.

Thus, it would be advantageous to provide a cellular system that couldservice a high concentration of users in a small geographic area, suchas users in a large office building, without the cumbersome handoverprocedures and frequency planning procedures that are currentlynecessary.

SUMMARY OF THE INVENTION

The present invention discloses a radio communication system comprisinga first base station for providing radio communication services to aplurality of mobile stations located within a first geographic area,said first base station transmitting a broadcast control channel (BCCH)for said first geographic area and at least a first capsule locatedwithin said first geographic area for transmitting traffic channels andreceiving traffic channels and random access channels (RACH) to and fromsaid plurality of mobile stations, said at least first capsule forproviding coverage over at least a first portion of said firstgeographic area. If a second capsule is provided the geographic areascovered by the first and the second capsules may significantly overlap.

Additionally, when a mobile station receiving a BCCH from said firstbase station transmits a RACH, each of said at least a first capsulereceives said RACH at a quality level. A controller determines saidquality level for each of said at least first capsule and allocates oneof said at least a first capsule for said first of said plurality ofmobile stations. The controller also builds a matrix indicatingpropagation loss between each of said at least a first capsule usingsaid quality levels. This radio communication system allows multiplemobile stations to transmit on the same frequency and the same timeslotwithin the first geographic area.

Inter-capsule handoff is accomplished, when available, by switchingassignment of the traffic channel for the first mobile station from thefirst capsule to the second capsule while maintaining the firstfrequency and the first timeslot of the traffic channel. Furthermoreeach of said at least a first capsule further comprises means forselectively providing a BCCH to an unservable portion of the firstgeographic area when the base station cannot provide a BCCH to saidunservable portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a conventional cellular system that isavailable in the prior art.

FIG. 2 is an illustration of a cellular system cell layout in accordancewith the present invention.

FIG. 3 is an illustration in block diagram form of a cellularcommunication system in accordance with the present invention.

FIG. 4 is an illustration of a cell in a particular configuration inaccordance with the present invention.

FIG. 5 is an illustration of a cell in a particular configuration inaccordance with the present invention.

FIG. 6 is an illustration of a particular cell configuration inaccordance with the present invention.

FIG. 7 is an illustration of a cell configuration in accordance with thepresent invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

The preferred embodiment includes a description of a radiocommunications system that utilises multiple cells to provide radiocommunication services to a plurality of mobile stations. In the cellsthat service high concentrations of mobile stations within a very smallgeographic area, such as large office buildings, there are provided aplurality of radio frequency (RF) capsules within the geographic areaserviced by the cell.

In addition to having a base site for providing a Broadcast ControlChannel (BCCH), the cell contains multiple RF capsules for transmittingand receiving traffic channels as well as receiving Random AccessChannel (RACH) bursts from a plurality of mobile stations in a portionof the geographic area serviced by the base site. The RF capsules do notprovide a BCCH, rather the BCCH is provided by the base station that isservicing the particular cell.

During call set up, a mobile transmits a RACH burst. This RACH burst isreceived by a plurality of RF capsules within the cell. A controller forthe cell site determines a quality level for each RACH burst received bythe RF capsules. The controller then allocates one of the RF capsules tothat particular mobile station. Thereafter, that mobile stationcommunicates to the cell site via the allocated RF capsule on a trafficchannel. A traffic channel is defined by at least a first frequency andat least a first time slot.

While the mobile station is transmitting to the allocated RF capsule,the neighboring RF capsules are monitoring these communications forquality. Again the controller responsive to the quality of the monitoredcommunications will switch assignment of the traffic channel for thisparticular mobile station from the allocated capsule to a newlyallocated capsule; preferably maintaining the chosen frequency and timeslot of the traffic channel. Thus, the hand-off goes unnoticed by theparticular mobile station.

Additionally, the controller builds a matrix indicating propagation lossbetween each of the plurality of RF capsules. The propagation losses aredetermined using the quality indications of the RACH burst received bythe individual RF capsules. In the preferred embodiment, the matrixcontains a list of average propagation delays between each of the RFcapsules. By monitoring the propagation loss between each of thecapsules, the particular RF capsule being used will know the effect ofthe frequency interference caused by its particular mobile station onthe RF capsules contained within the cell. This propagation lossinformation allows the controller to dynamically allocate frequencyre-use within the same cell.

Furthermore, in large buildings there are often block spots which areunserviceable by the base stations BCCH. In such situations, thepreferred embodiment provides a capsule for selectively providing a BCCHto an otherwise unservable portion of the geographic area to which thecell is supposed to provide communication service. For example, inbasements, or shielded areas, the selected RF capsule can repeat BCCHinformation broadcast by the base site into this unserviceable portionof the geographic area thus providing service for that particular area.

FIG. 2 is an illustration of a radio communication system including afirst cell 201, a second cell 203 and a third cell 205. A large building207 is illustrated by a square in which multiple RF capsules 209 areutilized within the first cell 201 to provide radio communicationservice to a high concentration of mobile stations within a very smallgeographic area. In this particular configuration, the first cell 201provides a BCCH and communication coverage for all mobile stationslocated within the first cell's geographic area illustrated by thehexagon. Within the building 207, there are five RF capsules which areset up to provide traffic channel communication between any mobilesubscribers. contained within their coverage area and to receive RACHbursts from the mobile subscribers. The details of controlling theseparticular capsules are illustrated in FIG. 3. You will note that RFcapsules coverage is illustrated by the irregular shaped circles 209that may severely overlap one and other without any repercussions on thequality of service provided to the mobile stations within that area.

FIG. 3 is an illustration of a base site in accordance with the presentinvention. The base site 300 contains a standard MSC (mobile switchingcentre) 301 that is interconnected to a conventional BSC (Base sitecontroller 303 via an A-line 305. Additionally, the base site 300includes at least one low cost Base Transceiver System (BTS) 307 and acontroller 309 for controlling the inter-capsule handoffs and assignmentof mobile stations to particular capsules as discussed below. The basesite 300 additionally includes an overlaid microcell transceiver 311 forproviding a BCCH for the entire cell area as well as traditionalcellular service in areas outside of the coverage of the RF capsules.There are a plurality of RF capsules 313 located in the base site 300for providing the service as illustrated in FIG. 2. In the preferredembodiment, the RF capsule is of a particularly small geometry and areconnected to the base transceiver system via a 270 Kilo Bit Per Second(KBPS) transmission line 315. Details of the system architecture may befound in co-pending patent application CE30155P filed on Dec. 4, 1995 byMotorola Ltd.

The overlaid microcell transceiver 311 continuously broadcasts a BCCH tomobile stations served in the fixed geographic area 201 of FIG. 2. Inresponse a mobile station desiring to set up a call transmits a randomaccess channel burst from its location. The RACH burst is received byany of the RF capsules 313 that are within distance to monitor the RACHburst from the mobile. Each of the RF capsules 313 receives the RACHbursts then either determines the quality of the received burst ortransmits the received burst back to the BTS 307. Controller 309 thenanalyses the results or alternatively determines the quality of thereceived RACH bursts from each of the RF capsules and determines themost appropriate RF capsule 313 to provide service to the mobilestation.

In the preferred embodiment the quality of the received RACH burst isdetermined by the received RSSI and the effective bit error rate. The RFcapsule 313 which provides the highest quality is chosen to provideservice to the RF capsule. If the quality of all of the RACH burstsreceived are less than a given threshold, then the radio communicationservice will be provided by the overlaid micro cell transceiver 311 in atraditional cellular manner.

The main reason for utilizing capsules instead of traditional cells isto reduce the handover overhead and to improve the re use factor from aconventional cellular system. A list of known good channels ismaintained by the controller 309. FIG. 4 is an illustration of a cellutilizing the base site 300 of FIG. 3. Here the microcell coverage area401 is provided by the overlaid microcell transceiver 311 in atraditional cellular manner. Additionally, the overlaid microcelltransceiver 311 broadcasts the BCCH for all RF capsules 403 containedwithin the geographic area of coverage area denoted by 401. In thisparticular example 5 RF capsules 403 are illustrated. These RF capsules403 provide traffic channel transmission and reception as well as RACHburst reception are controlled by a controller 309 of FIG. 3.

In FIG. 4, there are two mobile stations 405 and 407 within the cellarea 401 each being served by an independent RF capsule 403. Both mobilestations are utilizing frequency 3 and time slot 5 to broadcast theirtraffic channels back to their respective RF capsules. This can beprovided because of the technique discussed below of known propagationloss between the RF capsules. As the mobile station 405 moves towardsthe boundary of its present RF capsule 403, ideally the mobile stationwill maintain the same frequency and time slot when it switches serviceto another RF capsule 403. However, as both mobile stations 405, 407move towards the same RF capsule, only one of the mobile stations may beserved by the same frequency and time slot. The controller 309determines which one is better served by the frequency and time slot andswitches control from the first RF capsule to a second RF capsule 403.As illustrated in FIG. 5, the second mobile station 407 was determinednot to have priority over the first mobile station 405 and instead washanded over in a conventional manner. Thus, changing its frequency andor time slot with assistance of the mobile. As can be seen from theaforementioned example, the hand-offs can be provided in a non obtrusivemanner to mobile stations 405 and at worse case in a conventional manneras it would perform on mobile station 407.

As previously mentioned, one aspect of the present invention is toprovide a propagation map in the controller 309 so that propagationlosses between RF capsules 403 contained within a cell 401 are known.FIG. 6 is an illustration of a cell from which propagation maps may bedeveloped. Here, the cell 601 contains 5 RF capsules 603. When a mobilestation 605 transmits a RACH burst to initiate a call set up, each ofthe five RF capsules receive the RACH burst. The quality of the RACHinformation received by each of the RF capsules are gathered over aperiod of time. This information is then used to predict the probabilityof interference generated between the cells in a known manner. In thepreferred embodiment the controller 309 generates a matrix indicatingthe average propagation loss between each of the RF capsules 603.

After determining the average propagation loss between cells, one canpredict when the same frequency may be used within the cell 601 based onpropagation losses between RF capsules. For example, a mobile station islocated in RF capsule 1 of FIG. 6 and a second mobile station isoperating within RF capsule 4. If the average propagation loss betweenRF capsule 4 and RF capsule 1 exceeds a threshold, then the samefrequency may be re used within cell 601 for the two mobile stations. Inthe preferred embodiment the threshold is equal to 70 db.

FIG. 7 is an illustration of a radio communication system having 3 cells701 703, 705. Again, a large building is provided 707 with RF capsulesproviding cellular service to a large concentration of mobile stationswithin a small geographic area, as illustrated by RF capsules 709. Inthis particular case, there is an external interference caused byfrequency 1 as indicated by the large arrow 711 intruding into the cell701. This interference may be caused by an adjacent cellular system orsome other interference source. Traditionally, this interference couldonly be detected by the cell transceiver serving entire cell 701.However, in this particular case since RF capsules are being used toreceive smaller areas, the frequency F1 may be used in the areaillustrated by circle 713 of FIG. 7. The use of frequency F1 is possiblebecause of the isolation within the cell 701 created by the building 707and other isolation creators.

In certain situations, the serving micro cell 701 may not be able tobroadcast the BCCH efficiently throughout the entire area serviced by RFcapsules due to isolation and propagation losses in areas such asunderground structures and metal surround structures as is well known inthe art. In order to overcome this the RF capsules have the ability tobe switchably to repeat the BCCH data re transmitting it in a localisedisolated area in a traditional cellular manner.

Thus, as can be seen the foregoing discussion the radio communicationsystem of the present invention provides an improved hand-off for highconcentrations of mobile stations in a small geographic area as well aseasing of the cellular planning requirements of the traditional cellularsystems.

What is claimed is:
 1. A radio communication system comprising:a firstbase station for providing radio communication services to a pluralityof mobile stations located within a first geographic area, said firstbase station transmitting a broadcast control channel (BCCH) for saidfirst geographic area; at least a first capsule located within saidfirst geographic area for transmitting traffic channels and receivingtraffic channels and random access channels (RACH) to and from saidplurality of mobile stations, said at least first capsule for providingcoverage over at least a first portion of said first geographic area andconfigured to receive the RACH from the first of said plurality ofmobile stations at a quality level; and a controller comprisingfirstmeans for monitoring a communication quality of a first traffic channel,the first traffic channel having at least a first frequency and a firsttimeslot, second means for monitoring the communication quality of thefirst traffic channel through a second capsule, means, responsive to thefirst and second means for monitoring, for switching assignment of thetraffic channel from the first capsule to the second capsule whilemaintaining the first frequency and the first timeslot of the trafficchannel, and a matrix indicating, using said quality level, apropagation loss between said at least first capsule and the first ofsaid plurality of mobile stations, the controller allocating said atleast first capsule to the first of said plurality of mobile stations.2. The radio communication system in accordance with claim 1 wherein thesecond capsule is located within said first geographic area forproviding coverage over at least a second portion of said firstgeographic area, and wherein said first portion of said first geographicarea and said second portion of said first geographic area may overlapsignificantly.
 3. The radio communication system in accordance withclaim 2 wherein a first mobile station is transmitting on a first radiofrequency during a first timeslot and a second mobile station istransmitting on the same first radio frequency and the same firsttimeslot within the first geographic area.